The human anatomy includes many types of tissues that can either voluntarily or involuntarily, perform certain functions. After disease, injury, or natural defects, certain tissues may no longer operate within general anatomical norms. For example, organs such as the heart may begin to experience certain failures or deficiencies. Some of these failures or deficiencies can be diagnosed, corrected or treated with implantable medical devices.
Implantable medical electrical leads are used with a wide variety of these implantable medical devices. The medical leads may be configured to allow electrodes to be positioned at desired cardiac locations so that the device can monitor and/or deliver stimulation therapy to the desired locations. For example, electrodes on implantable leads may detect electrical signals within a patient, such as an electrocardiogram, in addition to delivering electrical stimulation.
An implantable cardioverter defibrillator (ICD) is a device that can be implanted in a patient's chest to monitor for and, if necessary, correct episodes of rapid heartbeat. If the heartbeat gets too fast (ventricular tachycardia), the ICD can stimulate the heart to restore a normal rhythm. In cases where the heartbeat is so rapid that the heart cannot effectively pump any blood (ventricular fibrillation), the ICD can provide an electric shock to “reset” the heartbeat.
Currently, ICD's use endocardial or epicardial leads which extend from the ICD housing through the venous system to the heart. Electrodes positioned in or adjacent to the heart by the leads are used for pacing and sensing functions. Cardioversion and defibrillation shocks are generally applied between a coil electrode carried by one of the leads and the ICD housing, which acts as an active can electrode.
A subcutaneous implantable cardioverter defibrillator (SICD) differs from the more commonly used ICD's in that the housing and leads are typically implanted subcutaneously such that the sensing and therapy are accomplished subcutaneously. The SICD does not require leads to be placed in the heart or in contact with the heart. Instead, the SICD makes use of one or more electrodes on the housing, together with a subcutaneous lead that carries a defibrillation coil electrode and a sensing electrode.
The implantable medical devices are typically battery powered and often utilize capacitors or other electrical charge storage components to hold an electrical output to be made available to a patient. Due to the nature of defibrillation therapy or other high voltage therapy, it is not practical for the implantable medical device to supply the energy upon instantaneous demand by drawing from the power source. Instead, additional circuitry is provided to transfer and store the energy from the power source to accumulate a desired voltage level.
The battery size is generally proportional to the charging time. Therefore, the quicker the desired charging time, the larger the battery needs to be. Nevertheless, there are numerous reasons including patient comfort and acceptance for a desirable to make the SICD as small as possible. Therefore large batteries are not desired and thus a balance must be struck between having a fast charging time and the size of the SICD.
Moreover, the placement of the SICD lead(s) and electrode(s) outside the heart presents a challenge to generating sufficient energy levels that are required to deliver appropriate therapy. As described herein, the present disclosure addresses the need in art to provide circuitry and techniques for generating appropriate electrical stimulation therapy in a SICD system.